Threaded Fittings general

Threaded joints probably represent the oldest method of joining piping systems. Like Socket Weld fittings, threaded fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.
The dimensional standards for taper pipe threads are given in ASME B1.20.1. That document gives all required dimensions including number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters.

Threaded piping is commonly used in low-cost, noncritical applications such as domestic water, fire protection, and industrial cooling water systems.
Threaded fittings are normally made of cast gray or malleable iron, cast brass or bronze, or forged alloy and carbon steel.
They are available in three pressure ratings: 2000lbs, 3000lbs and 6000lbs.

NPT Thread

All fittings on this page are described, are provided with NPT thread, ASME B1.20.1. The American National Pipe Thread Tapered, is the best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal. NPT has a tapered male and female thread which seals with Teflon® tape or jointing compound.

Dimensions of American Taper Pipe Thread, with Sealant Compound

DN NPT	Major Diameter mm	Tapping Drill Size mm	TPI	Pitch mm
1/16"	7.895	6.00	27	0.941
1/8"	10.242	8.25	27	0.941
1/4"	13.616	10.70	18	1.411
3/8"	17.055	14.10	18	1.411
1/2"	21.223	17.40	14	1.814
3/4"	26.568	22.60	14	1.814
1"	33.228	28.50	11.5	2.209
1 1/4"	41.985	37.00	11.5	2.209
1 1/2"	48.054	43.50	11.5	2.209
2"	60.092	55.00	11.5	2.209
2 1/2"	72.699	65.50	8	3.175
3"	88.608	81.50	8	3.175
3 1/2"	101.316	94.30	8	3.175
4"	113.973	107.00	8	3.175
5"	141.300	134.384	8	3.175
6"	168.275	161.191	8	3.175
8"	219.075	211.673	8	3.175
10"	273.050	265.311	8	3.175
12"	323.850	315.793	8	3.175

Types of Threaded Fittings by Class and Size

Description	Class Designation
Description	2000	3000	6000
Elbows 45 and 90 degrees Tees, Crosses, Coupling Half-Coupling, Cap	1/2 - 4	1/2 - 2	1/2 - 2
	1/2 - 4	1/2 - 2	1/2 - 2
	1/2 - 4	1/2 - 2	1/2 - 2
Pipe Wall	SCH 80 and XS	SCH 160	XXS

Plugs and Bushings are not identified. They may be used up through Class 6000 NPS

Fittings and Flanges for Threaded Systems

Elbow 90°

This Elbows make 90° changes of direction in the run of pipe.

Tee

This Tee makes 90° branch from the main run of pipe.

Cross

Crosses makes 90° branch from the main run of pipe.

Elbow 45°

This Elbows make 45° changes of direction in the run of pipe.

Full-coupling

Termed Coupling, joins pipe two pipe or to a nipple etc..

Cap (End Cap)

Seals the threaded end of pipe.

Half-coupling

The Half Coupling can be directly welded to the run pipe, to make a branch connection.

Square head plug

Seals the threaded end of fitting.

Hex head plug

Seals the threaded end of fitting.

Round head plug

Seals the threaded end of fitting.

Hex head bushing

Can be used to reduce a threaded fitting.

Union (MSS SP-83)

Unions are primarily used for maintenance and installation purposes. It is a screwed joint design and it consists of three interconnected pieces. Two internally threads and a centerpiece that draws the ends together when rotated.

Advantages and Disadvantages of Threaded Fittings

Advantages

Installation productivity is moderately high, and specialized installation skill requirements are not extensive.
Leakage integrity is good for low-pressure, low-temperature installations where vibration is not encountered.

Disadvantages

Rapid temperature changes may lead to leaks due to differential thermal expansion between the pipe and fittings.
Vibration can result in fatigue failures of screwed pipe joints due to the high stress intensification effects caused by the sharp notches at the base of the threads.
In hazardous piping systems threaded connections should be avoided, if possible. Their vulnerability to fatigue damage is significant, especially where exposed threads are subject to corrosion.

Remark(s) of the Author...

You knew that...?

At the smallest sizes, the amount of wall lost during threading actually equals approximately 55% of the original pipe wall.